1. Field of the Invention
This invention relates to a signal store with a signal-to-noise improving system which has particular, but not exclusive application, where still picture television video frame signals are to have the signal-to-noise ratio improved. A preferred embodiment of the invention has particular application in scientific environments, wherein a video signal involving still pictures having an inherently noisy nature can be improved.
2. Description of Prior Art
In scientific applications noise has hitherto been reduced in video picture frame signals by either a summing technique involving averaging a number of T.V. frames in order to suppress non-coherent signal components. The improvment in the signal-to-noise is proportional to the square root of the total number of frames involved in the averaging process. The necessary electronic hardware used to perform this method, if a considerable signal-to-noise improvement is to be obtained, requires that the memory be large and thus the resulting cost of the equipment is generally prohibitive. For example in a system using a gray-scale resolution of 8 bits, a signal-to-noise enhancement of 40 dB would require a memory size based on at least 21 bits per picture element.
A further method of reducing the signal-to-noise ratio has been by exponential smoothing: where an exponentially weighted moving average of A frames yield an ultimate signal-to-noise ratio improvement of .sqroot.2A-1 and allows a normalized image to be displayed while the signal averaging is progressing. This is referred to in "smoothing, forecasting and prediction of Discrete Time Series, by R. G. Brown, Prentice Hall, (1963) chaps. 7 and 8".
Theoretically the best result (in terms of both signal-to-noise improvement rate and ultimate value) which can be expected in any form of filtering technique is given by the summing algorithm i.e. enhancement=.sqroot..eta. where .eta. is the number of the frames.
For a system operating according to the summing technique it is a simple matter to calculate the memory size (i.e. bits per pixel) required to meet specific performance criteria. For example if a (voltage) enhancement factor of (say) 90 is required and the system gray-scale resolution is 8 bits then the size of the memory will be based on 21 bits per pixel and the total accumulation time (625/50 system) is approx. 6 minutes. If, in addition, a digital signal normalizer is required to produce a continuous display during the signal averaging process then the total system's hardware complexity would be considerable.